[ci] Read the GitHub event file as UTF-8 (#17896)

tests/script/test_helpers.py

@@ -79,6 +79,22 @@ def test_get_pr_number_from_github_env_event_file(
     assert result == "5678"
 
 
+def test_get_github_event_data_decodes_utf8_regardless_of_locale(
+    monkeypatch: MonkeyPatch, tmp_path: Path
+) -> None:
+    """The event payload is UTF-8; parsing must not depend on the platform
+    default encoding. On Windows the default is cp1252, which raised
+    UnicodeDecodeError as soon as a commit title carried non ASCII text."""
+    event_file = tmp_path / "event.json"
+    event_data = {"head_commit": {"message": "Answer UNPAIR with Response… é"}}
+    event_file.write_bytes(json.dumps(event_data, ensure_ascii=False).encode("utf-8"))
+    monkeypatch.setenv("GITHUB_EVENT_PATH", str(event_file))
+
+    result = helpers._get_github_event_data()
+
+    assert result == event_data
+
+
 def test_get_pr_number_from_github_env_no_pr(
     monkeypatch: MonkeyPatch, tmp_path: Path
 ) -> None: